1. Field of the Invention
The present invention relates to a vehicular a.c. generator having fans fitted to a bracket support type rotor and used for ventilating and cooling the generator and more particularly to a generator construction capable of suppressing the suction air noise caused by the fan.
2. Prior Art
FIGS. 1 and 3 are a sectional elevational and a side view of a conventional vehicular a.c. generator. As shown in these figures, the a.c. generator comprises a stator 1 formed of a stator core 2 and a stator coil 3, a rotor 4 formed of heteropolar magnetic pole cores 5, 6, the magnetic tooth portions of both being alternately projected in the circumferential direction, an exciting coil 7 held between the pole cores 5, 6, a rotary shaft 8 for securing the pole cores 5, 6, a pair of slip rings 9 held by the rotary shaft 8 via insulating sleeves, and fans 10, 11 respectively held by the pole cores 5, 6.
The generator further includes a front and a rear bracket 12, 13 for fixing the stator core 2 by means of clamping bolts 14. The front bracket 12 is provided with suction ports 12a for ventilation at one end and exhaust ports 12b in the outer peripheral portion thereof. Reference character m designates an external radius from the axial center of the suction port 12a. Moreover, the rear bracket 13 is provided with suction ports 13a at one end and exhaust ports 13b in the outer peripheral portion thereof. The rotary shaft 8 is support with the front and the rear bracket 12, 13 via respective bearings 15, 16. A pulley 17 is fitted to the rotary shaft 8 and used for transmitting engine torque to the rotary shaft 8 in order to rotate the rotor 4.
The generator further includes a rectifier 18 for converting a.c. current resulting from the a.c. voltage induced in the stator coil 3 to d.c. current, a heat sink 18a being used to radiate the heat of the rectifier, a voltage regulator 19 for detecting the generator voltage, controlling exciting current and adjusting terminal voltage to a predetermined value, a heat sink 19a being used to radiate the heat of the voltage regulator, and a brush holder 20 for urging brushes 28 held therein onto respective slip rings 9, the brush holding also performing a current collecting function.
As the rotor 4 in the a.c. generator described above rotates, the a.c. voltage is induced in the stator coil 3 and the power thus supplied causes it to generate heat. The stator unit 1 and the rotor unit 4 are ventilated and cooled by the rotating fans 10, 11.
The cooling air produced by the rotation of the fan 10 is sucked through the suction ports 12a of the front bracket 12 and discharged from the exhaust ports 12b as shown by an arrow A. The cooling air produced by the rotation of the fan 11 is sucked through the suction ports 13a of the rear bracket 13 and before being discharged from the exhaust ports 13b as shown by an arrow B, used for cooling the rectifier 18, the voltage regulator 19, the current collector unit and the stator unit 1.
FIG. 2 is an expanded view of a portion D of FIG. 1. The inner edge face of the front bracket 12 axially faces the vane 10a of the fan 10 in parallel with a small clearance g therebetween. The outside atmosphere is induced into the clearance g and flows therethrough as shown by an arrow C. If the clearance g is too large, the air to be discharged is caused to flow reversely and therefore it is inexpedient. The position of the external radius m from the axial center of the suction port 12a substantially corresponds to the center position of the radial length l of the vane 10a.
In the conventional vehicular a.c. generator described above, the clearance g between the vane 10a of the fan 10 and the inner edge face of the front bracket facing it is small and consequently ventilation resistance in the suction part in the external radial portion of the suction port 12a tends to become high. This causes noise offensive to the ears and constitutes a primary factor in raising an overall sound pressure level.
Further, in the conventional vehicular a.c. generator described above, vanes 10a of the fan 10 cross a circumferential portion where each of them and the suction part 12a overlap. Consequently, a collective frequency resulting from multiplying the rotating frequency by the number of vanes 10a and a frequency resulting from multiplying the rotating frequency by the number of suction ports 12a; that is, in the case of suction ports 12a in FIG. 4, the sound pressure due to the ventilating noise component of the 12-fold frequency of the suction port 12a tends to become conspicuous and offensive to the ears. In addition, this problem constitutes a primary factor in raising an overall sound pressure level.